[Frontend] Manual re-do of collapse/expand panes

open #90
Added code to MCTSplitter and MCTSplitPane to allow
toggling of CSS class 'resizing' on pane elements when
the user is actively using the splitter;
Added resize transition animation to split pane elements
when user not actively resizing;

platform/commonUI/general/res/sass/user-environ/_layout.scss

@@ -232,6 +232,11 @@
     }
 }
 
+.pane:not(.resizing) {
+    // Add transition CSS
+    @include trans-prop-nice-resize-w(250ms);
+}
+
 .pane {
     position: absolute;
     &.treeview.left {